1. Field of the Invention
The invention relates generally to lithography methods for fabrication integrated circuits and micro-devices and, more particularly, to nanoimprint lithography methods for forming, e.g., integrated circuits or ultra fine nanostructures on polymer thin films carried on a substrate.
2. Discussion of the Related Art
Nanoimprint lithography methods are newly developed lithography methods for fabrication of nanostructures with high-resolution, high-throughput and low-cost. The methods are based on an excellent replication fidelity obtained with polymers and combines thermo-plastic molding with common pattern transfer methods. Once a solid stamp (press mold) with a nanorelief on a surface thereof is fabricated, the solid stamp can be used for replication of many identical surface patterns. Therefore, the nanoimprint lithography methods can eliminate many limitations imposed upon conventional optical lithography, such as wavelength limitation, backscattering of particles in the resist and/or the substrate, and interferences. The nanoimprint lithography methods have potential application in the fabrication of, for example, micro electronic/mechanical systems, compact disk storage and magnetic storage systems, opto-electrical and optical devices, biological chips and microfluidic devices.
Hot embossing lithography method, which was proposed by Stephen Y. Chou in 1995, is an important nanoimprint lithography method. The hot embossing lithography method is typically based on pressing a mold into a thin film carried on a substrate to form a relief and then removing the compressed area of the thin film to expose the underlying substrate that replicates an obverse of a protruding pattern of the mold.
Referring to FIGS. 1A to 1D, a typical conventional hot embossing lithography method is shown. First, a mold 200 and a polymer thin film 300 formed on a substrate 100′ are provided. The mold 200 includes a main body 201, a number of relief features 202 having a desired shape and a number of recesses (not labeled), each recess being defined between a neighboring pair of relief features 202. The polymer thin film 300 can be deposited or coated on the substrate 100′ by any appropriate method, for example spin coating. Generally, the polymer thin film 300 includes a thermoplastic polymer, such as polymethyl methacrylate (PMMA). PMMA has a glass transition temperature about 105 degrees Celsius. When PMMA is heated to a temperature above the glass transition temperature thereof, PMMA is softened, has a low viscosity, and can flow.
The mold 200 can be patterned with the relief features 202 and the recesses with a lateral feature size of less than 25 nanometers. Such relief features 202 and recesses can be created, for example, by electron beam lithography, reactive ion etching (RIE) and/or other appropriate methods. In general, the mold is selected to be hard relative to the softened polymer thin film. The mold can be made of, e.g., metals, dielectrics, semiconductors, ceramics or their combination. Second, the mold 200 and the polymer thin film 300 carried on the substrate 100′ are placed in a vacuum chamber (not shown), and the mold 200 is aligning with the polymer thin film 300. Third, the mold 200 and the polymer thin film 300 are heated by a heater 400 to a temperature above the glass transition temperature of the polymer thin film 300. The mold 200 is pressed into the softened polymer thin film 300. As a result, the pattern of the relief features of the mold 200 are transferred to the polymer thin film 300, and a pattern conforming to the pattern of the mold 200 is formed on the polymer thin film 300. Fourth, the mold 200 and the polymer thin film 300 are cooled down, and the mold 200 is separated from the polymer thin film 300.
However, (1) the mold 200 is apt to be adhered to the polymer thin film 300 because of a strong adsorption force between the mold 200 and the polymer thin film 300. (2) Due to a difference in thermal conductivity of the materials, the mold 200 tends to cool down faster than the polymer thin film 300. Thus, the relief features 202 are liable to be jammed with the pattern formed on the polymer thin film 300. These phenomena will damage the pattern formed on the polymer thin film and, therefore, reduce a precision of the pattern on the polymer thin film or even destroy the pattern.
Solutions to solve the above problem have been proposed. For example, it is suggested to form a surface treated layer on a press surface of the mold 200 to reduce an adhesive force between the mold 200 and the polymer thin film 300. However, the sizes of the relief features on the press surface of the mold 200 are very small (less than 25 nanometers). Therefore, it is difficult to form an additional thin layer on the press surface, if even possible. Likewise, it is difficult to maintain a precise depth or width of the relief features on the press surface due to the additional thin layer. Furthermore, the surface treated layer is liable to release from the press surface during operation. Therefore, this solution is hard to practice, and the cost is expensive.
Therefore, what is needed is a hot embossing lithography method that is easy to operate, is relatively inexpensive, and has a high precision for pattern transformation.